Until log 6 the oven had no real front. No door, no hinges, just an opening made from flimsy metal (drums are made from around 0.8 to 1mm thick sheet metal).

I measured everything again and came up with the following dimensions of the front face:

So, I sat down and did some CAD'ing and ordered the parts in 8mm thick standard mild steel (S355JR, 1.0045).

I have no pictures of the single parts but here it is mounted to the front:

The frame is bolted to the drum with screws (which got a fair amount of copper paste before tightening them):

The handle to open and close the door is still a bit on the botchy side of things, but it works:

This is by the way one of the exceptions to building the oven from the cheapest/scrap parts. I wanted the front to look and feel professional, so I bought it plasma-cut and sandblasted. It was around 25€ I think.
But I used the inner cutout as the hatch (open in the above picture), which is not ideal because it has 3mm space all around, thus not closing exactly and wasting some heat...